Esoniq ESQ-1 Repair - Completely dead

Synth Repair
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Esoniq ESQ-1 Metal Case - Revision B – Was dead, no screen, now no output

I’ve had an ESQ-1 on the bench for a few weeks and thought I’d share where I’ve got to. It’s been an interesting one and I’ve hit a wall on a couple of things so any input would be welcome.

Came in completely dead — no display, no sound, nothing. Faded out before dying apparently, and someone had been in previously to replace the voice chips.

Power supply checked out fine against the service manual test points. CPU confirmed running on the oscilloscope, reset line releasing cleanly. So it was alive underneath, just not showing it.

Traced the dead display back to heat damage. A power transistor in the output stage had been running hot long enough to cause corrosion damage to the display board mounted directly above it. Cleaned it up, reflowed the affected joints, display came back without needing a controller chip replacement.

With the display back, more faults surfaced. Still working through them — op amp replacement on order, new keyboard ribbon cable on order.

The sound now is very very quiet, but it is there, i can change the instrument patch and that works, I think there is problem with the op amp - volume slider or even the data slider. I’m working though the op-amps trying to work it out.

The bit I’m also puzzling over is the keyboard crashes. Pressing most keys causes a soft reset and System Error 1. Same keys every time, crashes on key down. Already eliminated voice chips, ribbon cable, keyboard controller, matrix diodes and the switch board. The surviving keys produce sound in alternating groups of 4 chromatically. The crashes happen regardless of whether voice chips are fitted which makes me think there’s something in the scanning circuit itself rather than the voice hardware.

Has anyone seen this on a metal ESQ-1 and traced it to a specific chip? Would be good to hear if anyone has been down this road.

I’ve written the whole thing up in more detail here if anyone’s interested — https://www.drifttone.net/from-the-bench/ensoniq-esq-1/

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Read it. Good write up.

In all cases every synth from the 1980’s has time expired capacitors. The electrolyte in them has corroded out the internals making them unreliable even if tested ok. Tantalum capacitors especially can short with no visible signs causing loss of output.

But first I would suspect another op amp is dead somewhere in the audio path. Trace the audio path from the output and follow it back to the sound generating chips. Where the audio should be and isn’t is the fault.

I would not rule out a corroded or broken trace on the board somewhere. Those traces are just to small for a large board.

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this is where I am

Confirmed working: :black_small_square: PSU — all rails verified against service manual Figure 4 test points :black_small_square: CPU — booting, reset line confirmed, 8MHz clock on oscilloscope :black_small_square: Display — restored after cleaning heat damage from Q11 :black_small_square: Audio output stage — hum confirmed present :black_small_square: Q10 (MJE182) and Q11 (MJE172) — both tested good out of circuit :black_small_square: Matrix diodes — all tested good :black_small_square: Switch board — disconnected, ruled out :black_small_square: Voice chips — reseated, producing sound on working keys at previous session

Parts replaced / changed: :black_small_square: Keyboard ribbon cable — new cable fitted :black_small_square: TL084 op amp — TL074 fitted temporarily, TL084 on order :black_small_square: Solder joints reflowed throughout mainboard and display board :black_small_square: Corrosion cleaned from display board

Remaining faults:

1. Keyboard crashes Every keypress causes System Error 1 with new ribbon cable fitted. Original cable gives partial behaviour — some keys crash, some silent. New cable confirms the silent keys were a cable fault, not working voices. Root cause of crashes not yet identified. Bypass test on mainboard keyboard connector still pending.

2. Voice sound — regression noted Previously confirmed quiet but recognisable instrument sounds on some keys with different notes on different keys. Currently not hearing this — this is a backwards step from a previously confirmed state and needs investigating. Metronome also currently not sounding. Hum still present confirming output stage is alive.

3. Q10/Q11 temperature Both transistors warm to the touch but not burning hot as previously noted. Original components still fitted — replacement deferred until root cause of overheating is understood to avoid damaging new parts.

I agree it could well be a capacitor - likely downstream of Q10/Q11 as they are hot. (and were baking hot) but it could be so many other things

I’ve also been around about checking the diode junctions on the in/outs on more op-amps not a complete test but they all measured

Still lots of thinking to do!! Any comments would be very welcome especially around System Error 1 experience

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Great write-up. I’d try triggering the synth with MIDI to see if any of the crashing still occurs. This might help narrow down what’s causing the crash.

Getting a fresh set of EPROMs can help too. A corrupted EPROM can cause all kinds of weird bugs and crashes. I can’t guarantee it’s the issue but it’s good to rule it out.